1. Field of the Invention
This invention relates to a liquid crystal display device. More particularly, it relates to so-called active drive type liquid crystal display device with an excellent voltage maintaining property, which uses as its liquid crystal cell, a substrate arranged with a switching means such as an MIM element (a metal-insulator-metal field effect element), a field effect type thin film transistor (TFT) element or the like.
2. Description of the Prior Art
As conventional display devices using liquid crystals, twisted nematic liquid crystal display devices have been generally used, wherein a pair of opposed electrode substrates having an alignment film on each surface are positioned so that the aligning directions of the films intersect with each other at a given angle, and liquid crystals are filled between the opposed substrates in the state of twisted orientation. A twisted nematic liquid crystal display device wherein the twist angle of liquid crystals used is set at an angle of 90.degree. or more is also known as a super twisted nematic liquid crystal display device. Further, so-called guest-host type liquid crystal display formed by adding one or more dyes to liquid crystals is also known. Such display modes are referred to as twisted nematic liquid crystal displays hereafter.
As one of such devices, there is a twisted nematic liquid crystal display device for matrix display or TV display, wherein many pixel electrodes (which are usually transparent electrodes) and many MIM elements or TFT elements for switching said electrodes on and off are incorporated in the electrode substrates, which is also known as an active type twisted nematic liquid crystal display device.
On the other hand, various inorganic or organic materials have been used for alignment films formed on the electrode substrates in the above-mentioned liquid crystal display devices. Specifically, wholly aromatic polyimides which are prepared via a polyamic acid derived from an aromatic tetracarboxylic acid such as pyromellitic acid (or its dianhydride) and an aromatic diamine are used as a suitable material for forming the alignment film from the point of view that said polyimides have excellent chemical stability and thermal resistance.
However, In order to achieve a stable and good display in said active type liquid crystal display device, the voltage applied between each of the pixel electrodes is required to be maintained constant for a given period of time without loss. Namely the electric resistance between the pixel electrodes is desired to be as high as possible. For the purpose of evaluating the above property of the devices, for example, an expression of the following ratio is proposed, i.e. the voltage maintaining ratio P=(effective value of drain voltage for 1/60 sec/ source voltage). The expression indicates the extent of decrease for 1/60 second of a drain voltage generated by a pulse voltage application to a gate of a TFT in a liquid crystal device driven with a TFT element. When this ratio is utilized, it is desired to be as high as possible.
However, a liquid crystal device using an alignment film of the conventional wholly aromatic polyimide as mentioned above was difficult to exert stable and good display characteristics as an active type liquid crystal display device, because it was low in the voltage maintaining ratio P even if other conditions are appropriate.
The present invention has been made under such circumstances and intends to provide a twisted nematic liquid display device which in particular, has a high voltage maintaining ratio and is very suitable for an active type liquid crystal display device.